thE pAssAgE proBABly ContinuEs with A DisCussion oF ______.
A．thE rEAsons CrAtErs ArE DiFFiCult to stuDy
B．thE DiFFErEnt shApEs
C．somE FEAturEs oF lArgE CrAtErs
D．somE DiFFErEnCE in thE wAys smAll AnD lArgE CrAtErs wErE FormED